The present invention relates to solvent-processible conductive coatings, and more particularly to electrically conductive coatings of electrically conductive particle filled polysulfones.
Electrically conductive coatings are widely used in the electronics, defense and aerospace industries in conjunction with metals and plastics for electrostatic discharge electroless plating applications, in EMI/RFI shielding and in solder attach applications. Solvent processible polymer systems currently employed in this field include thermoplastic acrylics and polyesters, and thermosetting epoxies and acrylics, all of which have well known limitations. The thermoplastic systems, when filled with electrically conductive particles, exhibit excellent conductivity and are readily processible at low cure temperatures and short cure times. Unfortunately, these systems often exhibit limited chemical and temperature resistance properties which restrict their use.
Electrically conductive coatings used in capacitor construction, as well as in some of the previously mentioned applications, must be capable of forming a sturdy bond to solder (i.e., they must be "solderable") with good electrical contact. Solderability occurs through conductive particle-to-solder contact. The standard thermoplastic solderable coatings are acrylic or polyester based and decompose or degrade when heated to the 185.degree. to 250.degree. C. temperatures of soldering. While this decomposition is necessary to allow the solder to attach directly to the conductive particle filler, the decomposition or degradation is unpredictable and can undesirably spread beyond the solder attach site. Such undesired decomposition or degradation usually diminishes the electrical properties of the coating and the function of the entire article.
The thermosetting epoxies and acrylics, when filled with conductive particles, show improved chemical and temperature resistance in many applications. Such compositions often require prohibitive time/temperature cure profiles, have limited useful life and do not exhibit appreciable solderability.
The present invention finds particular utility as one of the conductive layers of tantalum capacitors. The layers surrounding the tantalum core of such capacitors most frequently are created by dipping the core in a series of baths containing a material to be deposited in an appropriate solvent. Thus, this "solvent processibility" is quite important in this field of use for conductive coatings.
Accordingly, it is an object of the invention to provide solvent processible electrically conductive coatings.
Another object of the invention is to provide such conductive coatings which exhibit good adhesion to a variety of substrates, form a sturdy bond to solder and are stable when exposed to high temperatures, i.e., those encountered during soldering.
A further object of the invention is to provide such conductive coatings which exhibit excellent electrical properties.